Food Allergy Testing
There are several different types of allergy tests that can be performed to determine whether an individual is allergic. It’s important to remember that reactions and history of reactions to certain substances are a clearer indicator of a potential allergy (or no allergy) than allergy testing alone. Following are some of the more common tests that are typically performed. You should discuss which test is appropriate for you or your child with an allergist or other medical professional.
Scratch or Skin Prick Testing
Scratch (also known as skin prick) testing involves making a tiny scratch or prick on the skin. A solution containing a small amount of the allergen in question is placed on the scratch. The area is watched for up to 20-30 minutes to see if a hive or wheal develops. If a wheal does develop, it is measured and the allergist determines whether there is a significant allergy or not.
Scratch testing can be done for a variety of allergies including food as well as environmental. Most tests are performed on the forearm or back. The tests are not particularly painful but may be uncomfortable. If hives develop they can become quite itchy.
You may need to stop any medications you are taking for up to 1-2 weeks before scratch testing, particularly antihistamines. Make sure to discuss your medications with your allergist prior to allergy testing.
Scratch testing is not for everyone. Individuals with a history of eczema or other sensitive skin issues may register with a false positive result. In some instances, a patient can be so severely allergic that the scratch test causes a more severe allergic reaction. In those cases, allergists typically recommend other forms of testing.
With intradermal testing, a small amount of the allergen solution is injected into the skin. This test is usually done when an individual is suspected to have an allergy but shows negative results with skin prick tests. The intradermal test is more sensitive than the skin prick test but shows more false positive results in people who do not have symptoms to that allergen .
Skin Patch Test
In a skin patch test, the allergen solution is placed on a pad that is taped to the skin for 24 to 72 hours. This test is used to detect a skin allergy called contact dermatitis.
Allergies can also be determined through a simple blood draw that is then sent to a lab. There are several types of blood tests available and which one you get depends upon your allergist. Blood tests are not as sensitive as skin tests, but are used for those who are unable to have skin testing done.
Enzyme-linked immunosorbent assay (ELISA, EIA), Radioallergosorbent Test (RAST), Immunoassay capture test (ImmunoCAP, UniCAP, or Pharmacia CAP) are all tests that detect specific antibodies (IgG in ELISA and IgE in RAST and ImmunoCAP) to suspected or known allergens. If an individual shows high levels of antibodies to peanut for instance, the test may indicate a peanut allergy. The higher the number in your specific IgE blood test report, the more your body has been sensitized to that allergen, which means the higher the likelihood of that allergen being a key cause of your symptoms.
IgE tests are typically scored on a scale from 0-6. Keep in mind that scores are not 100% representative of how severe the allergy or allergic reaction may be. Some individuals with high scores may have no or mild reactions and some individuals with lower scores may have severe reactions. Reactions can also change from one exposure to the next, therefore prior reactions are not an indicator of future reaction severity.
Blood testing is also not 100% accurate. There can be false positives and (rare) false negatives. Those with eczema will sometimes have positive blood test results, but no allergic reactions. The ELISA test measures IgG but does not differentiate between the four types of IgG antibodies. Some think IgG 4 may actually help to protect the body from harmful allergens. Since blood tests measure antibodies to allergens, they only indicate whether your body has been sensitized to that allergen. They do not indicate whether you will experience an allergic reaction from consuming or coming into contact with that allergen. It’s also important to note that those who have outgrown an allergy may still have positive IgE levels for years.
A food challenge is the most accurate test used to determine a food allergy. They can be held in a hospital or a doctors office, but should always be held under the supervision of a medical professional. Due to potential severe reactions, a food challenge is not right for all individuals.
During a food challenge the patient is given increasing amounts of the suspected food over a period of several hours. Depending upon the suspected allergy, the patient and the doctor; the food might be mixed into another food substance and/or a blind test may be done. This means the patient is not aware that they are consuming something they may be allergic to.
The medical professional watches the patient during this time for any signs of allergic symptoms/reactions. If the patient is able to consume a certain amount of the food in question without symptoms or a reaction, then it is determined that there is no allergy to the food. If the patient is able to consume some or none of the food before developing a reaction, then it is likely that that patient is allergic to that food.
Because all individuals and allergies are different, some allergic individuals are able to eat small amounts of foods they are allergic to but develop reactions when they’ve eaten too much. This is something that should be discussed with your allergist based on your own circumstances.
Component Resolved Diagnostic Test (CRD)
This new blood test (currently being used in Europe) goes deeper into allergens isolating specific proteins. CRD can find which proteins an individual is allergic too. Ara h1, h2 and h3 are peanut proteins most correlated to severe allergic reactions. However, there are other peanut proteins that will give a positive peanut allergy result on standard tests that in fact cause only mild reactions. This test is not currently FDA approved or widely available in the United States.
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